1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and more particularly, to a method of fabricating a ultra-frequency semiconductor device having a T-shape gate with a fine line width.
2. Discussion of Related Art
As communications technology advances, ultra-frequency bands from 1 GHz to 100 GHz are becoming of greater importance in communication system. A metal-semiconductor field effect transistor (MESFET) based on GaAs is generally used as a high-frequency semiconductor device. Since the GaAs has higher electron mobility than Si, and its transition time is shorter, it has excellent high-frequency characteristic and stands well against higher temperatures. Furthermore, the GaAs has a relatively wide energy gap. Thus, a low-power GaAs amplifying device operating at a room temperature, has a small amount of heat generation and leakage current. Thus, it also resists well against noise.
In addition to the MESFET, as a device used in a ultra-frequency integrated circuit, there is a high electron mobility transistor (HEMT), which can be also referred to as modulation doped field effect transistor (MODFET). The HEMT is an improvement over the MESFET, wherein undoped GaAs is used as its channel so as to remove impurity dispersion and increase the electron mobility.
Meanwhile, in order to improve the high-frequency characteristic in the GaAs MESFET and HEMT, reduction of their gate lengths is required. It is very difficult, however, to form a gate pattern having a length below 0.25 .mu.m using conventional I-line photolithography which is usually used for forming a mask pattern of above 1 .mu.m. To compensate for this problem, electron beam lithography is employed to form the gate pattern below 0.25 .mu.m. This method, however, increases the production cost, and decreases throughput due to its longer process time.